The purpose of a simulcast transmission system is to achieve wide area radio frequency (RF) coverage of a single frequency by simultaneous transmission from multiple geographical locations. Multiple transmitter sites are geographically located in such a way as to provide overlapping RF coverage, thus providing continuous signal reception over the combined coverage area for these transmitter sites. In order for a received simulcast signal to be intelligible, the remote site transmitters must all be modulated with substantially the same signal at substantially the same point in time. Typical industry standards for controlling modulation signals to the transmitters sites are .+-.0.1 dB amplitude stability, and .+-.2.5 microseconds signal delay. Due to these strict system requirements, the infrastructure of the simulcast transmission system between the transmitter sites must be carefully designed.
A common design methodology used for this infrastructure is a loop configuration. The loop configuration allows failure of a single link anywhere in the loop while maintaining continuity to all transmitter sites. When a loop system detects the failure of a link and redirects the loop to resolve continuity, the signal delay to some or all of the transmitter sites may be altered. As mentioned previously, the delays difference must be maintained to within .+-.2.5 microseconds for proper operation, highlighting the need for nearly instantaneous detection of changes in the system.
Loop microwave systems are designed with alarm units at each site which typically report the status of the microwave to a master alarm unit. Previous technology in simulcast systems would employ status lines directly wired to the master alarm unit, where the current status of the microwave system was logically derived. This method was effective, but severely limited to a hardware design which could not be easily upgraded when additional microwave sites were added to the loop for future communication system expansion. In addition, the direct wire method could not satisfy the following two conditions: 1) a simulcast system having multiple loops of microwave in the infrastructure, or 2) a single simulcast customer operating multiple systems (i.e., more than one audio source) on multiple loops.
FIG. 1A shows a simplified block diagram of a simulcast transmission system 100. In order for the simulcast network 106 to function properly, the system must be configured so as to account for transmission delays, audio path failures, and the addition and subtraction of repeater sites, some of which are directly coupled to RF transmitter sites, on the system. Today, this configuration process requires that an operator 110 manually adjust the system parameters through the use of user interface 102. The user interface then passes on the new delay parameters and path conditions to the simulcast network, via communication links 104. When problems in the audio path arose, such as broken or damaged communication links 104, the status lines would indicate this new condition to the alarm indicator 108. The condition would then be intepreted, or translated to a human-readable alarm by the alarm indicator 108, alerting the operator 110 to the fact that the system was inoperable. The operator 110 would then have to perform a manual, tedious process to address the new condition and bring the system back to an operational state. This often includes a manual search for a viable audio path which does not traverse the inoperable communication link, or links, which were detected by the alarm indicator 108.
Depending on the size of the simulcast transmission system, the aforementioned task may range from burdensome to nearly impossible. For a single loop system having only one audio source, there are generally only two data sets to consider: those for a clockwise audio path and those for a counter-clockwise audio path. In the same system the number of possible path conditions is equal to the number of repeater sites, or nodes, which may also be a small number (e.g., 3-10). As such, the operator's job of rectifying system problems remains fairly small. On the contrary, in larger, multiple loop systems, the number of alternate path conditions increases dramatically. As such, the simulcast transmission system may be inoperative for an hour or longer, during which time the operator finds himself frantically searching for one audio path which will render the system operational.
An alternate approach to this problem has been to utilize a computer-based polling routine which systematically measures the characteristics of each link in the system and makes equalization adjustments as necessary. This time consuming process, though, often results in the loss of in-process activity, as the system is rendered virtually unusable during these measurement periods. In the event of an emergency call being made through a particular repeater while it is being polled, the audio for that call would be unintelligible, at best; more likely, though, the call is lost altogether. This compromise of public health and safety is viewed as an undesirable, yet predictable, consequence of the polling method.
Accordingly, there exists a dire need for a system which is capable of automatically detecting system audio path problems, and reconfiguring that system to put it back into an operative state in a timely fashion. Additionally, there exists a need for an on-line, automatic audio path map generation scheme which can readily handle changes in system configuration so that system downtime is reduced to a minimum. The human-driven systems currently employed are becoming more inadequate as simulcast transmission systems become more complex.